Until a few years ago, shop-bought filter-tipped cigarettes comprised a filter made solely of one portion of cellulose acetate formed from a strip of cellulose acetate, which is stretched, impregnated with plasticizing additives, and rolled to form a cylindrical rod which is then wrapped in a sheet of paper material.
A filter made of a single portion of cellulose acetate is capable of blocking and retaining course particulate and moisture present in tobacco smoke, but fails to adequately block volatile substances in the smoke. For which reason, a cigarette filter has recently been proposed, in which a further filtering portion, comprising active-carbon granules, is interposed between two portions of cellulose acetate. Various tests, in fact, have shown the filtering portion of active-carbon granules to be highly effective in blocking and retaining volatile substances present in tobacco smoke; and the active-carbon granules may be combined with additives to selectively block targeted substances in the smoke.
U.S. Pat. No. 3,066,681A1 discloses a cigarette comprising a cylindrical body of tobacco, a wrapping sheet around the tobacco, and a cylindrical cartridge proximate one end of the tobacco body and in alignment therewith including a liquid-impregnated porous mass of filter material, a shell having a liquid proof cartridge wrapper around the mass of filter material, and means extending across at least one end of the cartridge wrapper including a liquid proof and smoke-permeable membrane.
GB1329956A discloses a tobacco-smoke filter comprising at least three different filtering agents, at least one of which agents is primarily intended to remove smoke components with a particle diameter of more than 0.1 micron, and at least one other of which agents is a polar adsorption agent intended to remove chemically polar smoke components with a particle diameter of less than 0.1 micron. The specified polar adsorption agent is porous magnesium silicate of 0.1-2.0 mm particle size, and the agent for removing particles of more than 0.1 micron diameter may be a wad of cellulose acetate or crepe paper.